Such a method is known from U.S. Pat. No. 5,759,623 which discloses a pretreatment step of supersaturating an iron-based surface with carbon prior to the deposition of a diamond coating. The deposition involves a Chemical Vapor Deposition (CVD) process.
For a relatively long time it has been known how to deposit from a gas phase a polycrystalline diamond coating on a selected substrate. The substrate is exposed to a gas mixture comprising at least a carbonaceous compound which is activated. Often methane is used together with hydrogen in such a gas mixture. The activation of the gas mixture generally requires severe conditions such as high gas temperatures of well above 2000° C. or microwave radiation. The substrate is generally kept at an elevated temperature (typically 900° C.) and comprises a well chosen ceramic such as silicon or a relatively expensive metal such as molybdenum.
Given the superior properties of diamond in terms of hardness, wear resistance, and chemical stability, it is advantageous to apply a CVD diamond coating to substrates which are in use exposed to severe conditions.
PCT International Publication No. WO 99/31292 and EP 0 984 077 both disclose a cemented carbide or tungsten carbide (WC)-based hard metal substrate onto which a CVD diamond coating has been deposited. Such substrates are relatively expensive.
EP 0 320 657 describes a method for coating metal substrates with CVD diamond. Such substrates include refractory metals, such as Rhenium, Tantalum and Tungsten onto which diamond deposition without the use of an interlayer has been shown to be possible. Also these substrates are relatively expensive.
Given the many applications of iron and steel and the relatively low price of iron and steel, it is desirable to have a CVD diamond coating on a steel or iron substrate. Such a combination of a coating and a substrate will provide a cost-effective substitute for tools fabricated out of materials such as cemented carbides which are often used in industrial applications. The CVD process that is used in the present invention, is a particular suitable way of applying diamond and is well documented. See for example: “CVD diamond films: nucleation and growth”; S.-Tong Lee, Zhangda Lin and Xin Jiang, Materials science and Engineering R (Reports: A review journal), 25 (1999) 123–154, which document is incorporated herein by reference to provide process equipment and process conditions for the CVD process used in the process of the invention.
However, as said, during the CVD process the substrate temperature is kept at a high temperature. Within a temperature range from this high temperature down to room temperature there is a very high difference in thermal expansion coefficient between iron or steel and diamond. This leads to high interface stresses during cooling down from the deposition temperature. As a result, the coating may delaminate from the surface or give rise to other disadvantageous effects.
It is for instance possible that the coating only partly delaminates at patches on the interface, therewith releasing stress and reducing adherence to the substrate.
If a coating remains completely attached to the surface, the interface stress within the coating can be enormously high enhancing the likelihood that the coating will delaminate during subjection to practical applications.
The residual stress in a coating can be measured as indicative of the interface stress. Ideally, the deposition process results in a continuous diamond coating with a low residual stress. In practice, this has proven to be a difficult task.
Apart from stress related problems, the deposition of diamond on iron-based substrates and especially steel is hampered by the fact that materials containing iron phases at the surface favor at deposition conditions formation of graphite instead of formation of diamond. This, of course further inhibits formation of a well adhering diamond coating on steel. Moreover, the high solubility of carbon in iron phase containing substrates appears to hinder formation of CVD diamond coating.
In attempts to solve these problems, interlayer systems have been developed with attempts to 1) suppress diffusion of iron to the substrate surface, 2) suppress diffusion of carbon into the iron phase, and 3) accommodate for and/or reduce the high interface stresses.
Reports have been published on the use of overlay coatings, which are deposited on top of the steel substrates prior to CVD diamond deposition. These overlay coatings may have a thickness of the order of several micrometers and are mainly produced by Physical Vapor Deposition (PVD) techniques. Some examples involve the use of molybdenum, nickel, titanium nitride and chromium nitride interlayers.
None of these overlay coatings have been proven to be very successful as an abrupt change in the elastic, thermal and/or mechanical properties is present at the interface with the deposited diamond coating. These changes lead to high interface stresses, which often result in delamination during cooling down from deposition temperature and if not, ultimately to premature failure during use in industrial applications. Also the interlayer/steel interface is abrupt, enhancing the likelihood of failure of this layer system at a depth further away from the diamond coating.
In a further attempt to overcome at least some of these problems, a diffusion modified overlay coating has been employed. Hereto, a chromium layer was deposited on a steel substrate by means of electroplating and this chromium layer was subsequently nitrided by passing a gas mixture comprising nitrogen. Also applying a chromium layer to a steel substrate subsequently followed by diffusion of the chromium into the steel has resulted in a interlayer which allows for CVD diamond coating of steel as reported by.
This approach has within the overlay coating resulted in—towards the iron-based substrate—gradually changing properties such as elasticity, hardness and thermal expansion of the material, leading to a slightly better accommodation of the thermal stress during the CVD diamond deposition process. However, there remains a large mismatch in the thermal and mechanical properties at the interface of a diamond coating and the substrate surface, as well as an abrupt change in properties at the chromium/steel interface introducing weak interfaces in the layer system. Also very high residual stresses (indicative of high interface stresses) have been observed within the diamond films deposited on this layer system, limiting the scope of applications.
It should further be noted that this method of producing an interlayer system includes three steps for obtaining a CVD diamond coating on steel. Each step is carried out in a different reactor, making the method complicated and costly. These steps are: 1) electroplating, 2) nitriding and 3) depositing diamond.
High temperature diffusion chromizing as a successful method for CVD-diamond coating of steel by S. Schwarz et al. in Diamond and Related Materials 11 (2002) 757–762.
Previously identified U.S. Pat. No. 5,759,623 discloses supersaturation of an iron based substrate with carbon by heating the substrate and exposing it to a carbon atom donor substance. It is suggested that by in-diffusion of carbon a diffusion barrier is formed in the iron based substrate. A black carbon deposit is formed on the surface of the substrate and after removing this deposit the substrate is suitable for diamond to be deposited on the surface thereof.
However, at least the following two problems remain. During the CVD process, carbon already present in the iron-based substrate diffuses further away from the surface, thereby slowly dissolving the diffusion barrier at a side adjacent to the matrix of the iron-based substrate. This allows for more iron to diffuse to the surface, hampering the formation of diamond during deposition. Iron may also catalytically aid the transformation of diamond to the more stable form of graphite. A further detrimental result of the presence of iron at the interface with the diamond coating that may occur is premature delamination of the diamond coating in use due to conditions where the diamond coating is subjected to heat. Another problem is that a large mismatch in properties is still present between diamond and the surface of the supersaturated iron based substrate. The Vickers hardness of carburized steel surfaces is reported to be about 1,000, whereas the diamond grains themselves have a VHN (VHN=Vickers Hardness Number) of 10,000.